Why a separate DAC?

There are several factors into building into a quality sounding dac ,
it is not just about the dac chipsets. My BurrBrown 1792 chipsets in my Belcanto  2.7 dac sound betterthen many that sell for much more money the powersupplies,and isolation is very important ,
as well as the word clocks which mine have Dual Femto clocks 
which is class leading coming from a designer that has over 25 years working with digital ,as well as having his roots in Vacuum tubes and designs his Sonics with that in design .by having a seperate dac it can have the isolation needed as welssthe parts quality . http://www.belcantodesign.com/home/eone/dac-2-7/

OCG HiFi (you tube channel) just compared 15 different DACs from $850- 23k+ 

I am currently using a Audio Alchemy DDP-1 (preamp/dac) + AS 5 (outboard power supply) It's a freakin bargain. No loner available new as AA introduced a new line, but I have seen the combo (the PS 5 is a must) used for $1700. Jump on that

+1 Mhdt Labs Orchid R2R

Might want to check out the Mirror Audio Tubadour III. $1500. Tube output stage.

If you're looking for a separate DAC, I would echo those that recommend the MHDT Labs Orchid.  I'm listening to mine right now and it is my favorite out of some 10 or so that I have if you count all the CD players, and receivers/integrateds with DACS.